Clause (8.13.1) - A vent or chimney serving a single appliance shall provide effective venting and shall be sized
(a) so that its effective area is not less than that of the draft-control device outlet or the flue outlet; or
(b) in accordance with good engineering practice, such as by the use of
(i) Table C.1, C.2, C.5, or C.6 of Annex C for a draft-hood-equipped or a fan-assisted Category I appliance; or
(ii) engineered venting tables acceptable to the authority having jurisdiction.
Annex C (C.2.2) - The vent tables included in this Annex apply to vents and chimneys internal to the structure below the roof line. Exterior chimneys or vents not enclosed by the structure or a chase below the roof line can experience continuous condensation, depending on locality. A chimney with one or more sides exposed to the outside of the structure shall be considered to be an exterior chimney. A Type B vent or a certified chimney lining system passing through an unused masonry chimney flue shall not be considered to be exposed to the outdoors. The DP column shall be used to determine the capacity of a venting system within a building constructed in accordance with Clause 8.2.1.
HVAC TECH GROUP - Director's Advisory FS-065-05 https://hvactechgroup.com/article.php/20070429181100239